Optoelectronic devices using organic materials are becoming increasingly desirable in a wide variety of applications for a number of reasons. Materials used to construct organic optoelectronic devices are relatively inexpensive in comparison to their inorganic counterparts thereby providing cost advantages over optoelectronic devices produced with inorganic materials. Moreover, organic materials provide desirable physical properties, such as flexibility, permitting their use in applications unsuitable for rigid materials. Examples of organic optoelectronic devices comprise organic photovoltaic cells, organic light emitting devices (OLEDs), and organic photodetectors.
Photovoltaic devices convert electromagnetic radiation into electricity by producing a photo-generated current when connected across a load and exposed to light. The electrical power generated by photovoltaic cells can be used in many applications including lighting, heating, battery charging, and powering devices requiring electrical energy.
When irradiated under an infinite load, a photovoltaic device produces its maximum possible voltage, the open circuit voltage or Voc. When irradiated with its electrical contacts shorted, a photovoltaic device produces its maximum current, I short circuit or Isc. Under operating conditions, a photovoltaic device is connected to a finite load, and the electrical power output is equal to the product of the current and voltage. The maximum power generated by a photovoltaic device cannot exceed the product of Voc and Isc. When the load value is optimized for maximum power generation, the current and voltage have the values Imax and Vmax respectively.
A key characteristic in evaluating a photovoltaic cell's performance is the fill factor, ff. The fill factor is the ratio of the photovoltaic cell's actual power to its power if both current and voltage were at their maxima. The fill factor of a photovoltaic cell is provided according to equation (1).ff=(ImaxVmax)/(IscIoc)  (1)The fill factor of a photovoltaic is always less than 1, as Isc and Voc are never obtained simultaneously under operating conditions. Nevertheless, as the fill factor approaches a value of 1, a device demonstrates less internal resistance and, therefore, delivers a greater percentage of electrical power to the load under optimal conditions.
Photovoltaic devices may additionally be characterized by their efficiency of converting electromagnetic energy into electrical energy. The conversion efficiency, ηp, of a photovoltaic device is provided according to equation (2) where Pinc is the power of the light incident on the photovoltaic.ηp=ff*(IscVoc)/Pinc  (2)
Devices utilizing crystalline or amorphous silicon dominate commercial applications, and some have achieved efficiencies of 23% or greater. However, efficient crystalline-based devices, especially of large surface area, are difficult and expensive to produce due to the problems in fabricating large crystals free from crystalline defects that promote exciton recombination. Commercially available amorphous silicon photovoltaic cells demonstrate efficiencies ranging from about 4 to 12%.
Constructing organic photovoltaic devices having efficiencies comparable to inorganic devices poses a technical challenge. Some organic photovoltaic devices demonstrate efficiencies on the order of 1% or less. The low efficiencies displayed in organic photovoltaic devices results from a severe length scale mismatch between exciton diffusion length (LD) and organic layer thickness. In order to have efficient absorption of visible electromagnetic radiation, an organic film must have a thickness of about 500 nm. This thickness greatly exceeds exciton diffusion length which is typically about 50 nm, often resulting in exciton recombination.
It would be desirable to provide organic photovoltaic devices which display increased efficiencies in converting electromagnetic energy into electrical energy.